1. Field
The following description relates to detection of optical signals, and more particularly, to an apparatus and method for detecting remodulated On-Off Keying (OOK) signals.
2. Description of the Related Art
On-Off Keying (OOK) is used to convert binary signals into signals with specific amplitudes for communications. OOK signals have been widely used for optical communications and applied to general digital devices as they have simple waveshapes and are easy to intuitively understand.
A Wavelength-Division-Multiplexed Passive Optical Network (WDM-PON) uses, as a light source, a Reflective Semiconductor Optical Amplifier (RSOA) or a SOA-REAM (Reflective Electro-Absorption Modulator). When an RSOA is installed in a subscriber side, the RSOA amplifies and remodulates OOK signals. At this time, the amplitude of a remodulated signal “1” depends on whether a signal received by the subscriber side has a level of “0” or “1”.
A general method of detecting OOK signals is based on a mathematical model. In detail, a mathematical model is established using the amount of noise existing in a “0”-level signal, the amount of noise existing in a “1”-level signal and the amplitude of the signal. Then, based on the mathematical model, a proper threshold value is decided or maximum likelihood detection is performed. However, as described above, since a remodulated OOK signal “1” is dependent on the amplitude of its original signal before remodulation, applying an existing mathematical model to detect OOK signals fails to obtain optimum performance.
Also, a device which uses a fixed threshold in detecting OOK signals has difficulty in setting the threshold to an optimized value. Furthermore, in the case of a Time Division Multiplexing (TDM) network where signals vary over time, the detection based on a fixed threshold is limited in terms of performance.